It is well known that in drilling boreholes in the earth, such as deep wells for oil and gas exploration, precise control of the path followed by the well is extremely difficult, so that it is virtually impossible to know the exact location of the well at a given depth. For example, a drilling tolerance of plus or minus one quarter of a degree will allow the bottom of a 10,000-foot well to be positioned anywhere within a circle 100 feet in diameter, and numerous other factors can increase the deviation. This is not of particular concern in many drilling operations, but if drilling precision is necessary, as where a borehole is to be drilled precisely to intersect a target location, is to be drilled so as to avoid an existing well, or is to be drilled to be parallel to an existing borehole, such variations can cause severe difficulties. One example of the need for precision drilling occurs in the situation where it becomes necessary to drill a relief well to intersect an existing deep well, as in the case where the casing of the deep well has ruptured and it becomes necessary to plug the well at or below the point of the rupture to bring it under control. In order to do this, the relief well must be drilled to intersect the original well at the desired level, and since such ruptures, or blowouts, often produce extremely hazardous conditions at the surface in the vicinity of the original well, the relief well usually must be started a considerable distance away from the original wellhead and drilled at an incline down to the desired point of intersection.
Because the same problems of control of the direction of drilling that were encountered in the original well are also encountered in drilling the relief well, the location of the relief well borehole also cannot always be known with precision as the relief well is being drilled; accordingly, it is extremely difficult to determine the distance and direction from the end of the relief well to the desired point of intersection on the target well. In addition, the relief well usually is very complex, compounding the problem of knowing exactly where it is located with respect to a target that may be 10 inches in diameter at a distance of thousands of feet below the earth's surface.
Numerous early attempts were made to solve the problem of guiding a relief well to accurately intersect a target well. Some utilized surveying techniques to locate the relief well with respect to a target well, but such survey techniques are not capable of providing accurate data concerning the relationship of the relief well to the original well until the relief well has approached very near the original well. Magnetic gradient ranging equipment can be used with considerable accuracy at close range; however, it has been found that outside a radius of a few tens of feet, such systems are usually inadequate.
In an attempt to extend the distance at which accurate information can be obtained, a variety of electrical well logging techniques have been used which treat the target well as an anomaly in the geologic structure of the earth surrounding the relief well. Some of these systems are directed to the measurement of the apparent resistivity of the earth across a pair of electrodes but, since no directionality is given by this method, it is ineffective for directing a relief well with respect to an existing well.
In addition, there have been attempts to obtain similar data through the use of electromagnetic prospecting, where induction sensing coils mounted at right angles to each other are used in conjunction with other conventional well logging systems to determine the probable location of a target. However, such systems do not suggest the possibility of locating relatively small targets such as well bores.
Other systems have been developed for directing a second well with respect to a first well by the use of sonic detectors responsive to the sound produced by fluids flowing out of a ruptured well formation. However, such systems do not operate when there is no sound emanating from the target well, and, in addition, do not provide the required degree of directional and distance accuracy. Another system uses a signal transmitter in one well and a signal receiver in the other well, wherein sound waves or magnetic fields may be used as the signals. In such a system, however, the target well must be accessible so that the signal source can be placed in one well and the receiver in the other, and is not effective where the target well is not open.
Many of the difficulties outlined above were overcome in the prior art by methods and apparatus disclosed, for example, in U.S. Pat. Nos. 4,323,848, 4,372,398, 4,700,142, and 5,512,830, all issued to Arthur F. Kuckes, the applicant herein. In accordance with these patents, an electric current flow is produced in a target such as the casing of a target well by injecting a low frequency alternating current into the earth surrounding the target well through the use of an electrode located in a relief well, or borehole. This current flow extends between the downhole electrode and a second electrode that may be located at the earth's surface in the vicinity of the head of the relief well. A portion of the injected earth current finds a path of least resistance through the casing or other current-conducting material in the target borehole, and the resulting concentration of current produces a characteristic magnetic field surrounding the target well which can be detected by an AC magnetic field sensor such as that described in U.S. Pat. No. 4,323,848, or by multiple sensors, as described in U.S. Pat. No. 5,512,830. These sensors are extremely sensitive to very small magnetic fields, and accurately detect the vectors of magnetic fields produced by currents flowing in well casings located a considerable distance away from the relief borehole.
The vector signals obtained from the AC magnetic field sensors, in accordance with the aforesaid patents, permit calculation of the direction and distance to the target well casing with respect to the location of the AC magnetic field sensor in the relief well. This information can be used to guide further drilling of the relief well. Thus, as the relief well approaches a desired depth, its approach to the location of the target well can be guided so that the target well is intersected at the desired depth below the earth's surface in a rapid and effective manner. This method of guiding a relief well to intersect with a target is a homing-in process, wherein multiple measurements—often after every 50 feet of drilling—are made as the relief borehole approaches the target. Since the drill string for the relief well must be pulled for each measurement, the drilling of a relief well becomes very expensive, especially in off-shore drilling, wherein more time may be spent measuring than is spent drilling.
The foregoing systems are widely, and successfully, used; however, the need for time-consuming periodic withdrawals of the drill string so that suitable sensors and electrodes for generating the ground current can be lowered into place to obtain distance and direction measurements from the relief well is a drawback, since a drilling rig operation can cost upwards of $500,000.00 per day in offshore drilling operations. Accordingly, a method and apparatus for making such measurements without the effort and expense of pulling the drill string is needed.
Another difficulty encountered in typical borehole drilling operations such as those described above is that the path of the borehole, which may be a relief well as described above, is tracked during drilling by a “measurement while drilling” (MWD) instrument that is mounted near, but not at, the bottom of the drill string. Usually, a drill string consists of a series of steel tubes, each about 10 meters in length and connected end-to-end. Connected near the bottom end of the drill string is a non-magnetic section which carries the MWD instrument, and connected below that is a hydraulic drilling motor having a bent housing to which the drill bit is connected via a drill shaft, with each of the non-magnetic section and the bent housing being about 10 meters in length. As a result of this, the MWD instrument is typically located 10-20 meters above the face of the drill bit, so that when magnetic field measurements are made with the drill string in the relief well, they are actually made a considerable distance from the drill bit, introducing a significant error in determination of the relative distance and direction of the target with respect to the drill bit. This greatly increases the difficulty of accurately controlling the location of the borehole being drilled with respect to the target.
Accordingly, there is a need for a measurement system that will significantly increase the accuracy of distance and direction calculations in drilling, without withdrawing the drill string from the borehole being drilled.